Communication service providers (e.g., wireless, cellular, internet, etc.) and device manufacturers are continually challenged to deliver value and convenience to consumers by, for example, providing compelling network services, applications and contents. One area of interest has been the development of social networking services and other services for making connections and sharing data, contents or resources among users. One exemplary service is that various personal/private data (such as health records, exam results, work schedules, etc.) of a user could be saved in a data center, for example offered by a cloud computing service provider (CSP), to release the storage and maintenance burden of the user equipment. In an example scenario, a user's health records can be saved and maintained in a data center offered by a CSP for sharing among multiple other users. Obviously, the user would like to permit only appropriate users to access this personal data. For example, when a health treatment of the user in a hospital is needed, a doctor in this hospital can be granted a right to access the user's health records stored in the data center. However, after the user left the hospital, for example after the user was cured or transferred to another hospital, the user may want to revoke the granted access right of the doctor due to dissatisfaction (i.e., trust in him/her is decreased), and/or may want to permit another doctor in the another hospital to access his/her health records. In addition, the data center or the CSP may be semi-trusted. For example, the CSP may be curious to know or maliciously disclose the user's personal data.